1. Field of the Invention
The present disclosure relates to a power conversion system and a power converter for converting a DC power generated by a solar cell into an AC power.
2. Description of the Related Art
In the related art, maximum power point tracking (MPPT) control is often used to control a solar cell to acquire generated power maximally. MPPT control allows the operating voltage (output voltage) of a solar cell to vary in predetermined step sizes to search for the maximum power point of the solar cell. MPPT control is usually executed by a step-up converter provided between the solar cell and the inverter. The generated power at the maximum power point is output to the inverter via a DC bus. The inverter converts the generated power (DC power) input via the DC bus into an AC power and outputs the AC power to the commercial power system (hereinafter, simply referred to as the system).
The inverter controls the voltage on the DC bus to be higher than the voltage of the system and outputs the power to the system. In this process, the closer the voltage on the DC bus to the voltage of the system, the smaller the power conversion loss.
Meanwhile, the DC bus is connected to the output of the step-up converter, and the input voltage of the solar cell cannot be equal to or higher than the bus voltage. In other words, the operating voltage point of the solar cell is limited to be equal to or lower than the bus voltage. This means that the operating point of the solar cell cannot be controlled to an optimal value when the optimal operating point of the solar cell is in a zone equal to or higher than the bus voltage.
One conceivable approach is to control the optimal operating point of the solar cell such that the bus voltage is maintained at a voltage that cannot be where the optimal operating point is found. For example, the operating point may be maintained at the maximum voltage value (e.g., 400 V) of the solar cell panel, but that would worsen the power conversion loss in the inverter.
There is also proposed a method of reducing power conversion loss by controlling the bus voltage arbitrary while MPPT control is being executed to meet a condition requiring that the bus voltage is equal to or larger than the system voltage (refer to Japanese Unexamined Patent Application Publication No. 2013-90364, for example).
In the method proposed above, a determination is made as to whether the bus voltage meets a reference value (i.e., whether the bus voltage is equal to or higher than the system voltage) based on the input voltage of the solar cell. However, because there are semiconductor devices such as reactors and diodes between the input of the solar cell and the DC bus, a voltage drop occurs due to those semiconductor devices and an error is created between the input voltage of the solar cell and the bus voltage.
To compensate the error, it is necessary to increase the reference value by a margin corresponding to the error, which increases the bus voltage and reduces the loss reduction effect. Since the operation of the step-up converter and the inverter is determined based on the result of comparing the input voltage of the solar cell with the reference value, it is necessary to send the input voltage of the solar cell from the step-up converter to the inverter by way of communication if the step-up converter and the inverter are separately controlled.